In the past considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are rapidly being depleted, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal, natural gas and other hydrocarbons, or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels, such as by the electrolysis of water using renewable energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable hydrogen storage medium. Conventionally, hydrogen has been stored in a pressure vessel under a high pressure or stored as a cryogenic liquid, being cooled to an extremely low temperature. Storage of hydrogen as a compressed gas involves the use of large and bulky vessels.
Additionally, transfer is very difficult, since the hydrogen is stored in a large-sized vessel; amount of hydrogen stored in a vessel is limited, due to low density of hydrogen. Furthermore, storage as a liquid presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below −253° C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen.
Alternatively, certain metals and alloys have been known to permit reversible storage and release of hydrogen. In this regard, they have been considered as a superior hydrogen-storage material, due to their high hydrogen-storage efficiency. Storage of hydrogen as a solid hydride can provide a greater volumetric storage density than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. Solid-phase metal or alloy system can store large amounts of hydrogen by absorbing hydrogen with a high density and by forming a metal hydride under a specific temperature/pressure or electrochemical conditions, and hydrogen can be released by changing these conditions. Metal hydride systems have the advantage of high-density hydrogen-storage for long periods of time, since they are formed by the insertion of hydrogen atoms to the crystal lattice of a metal. A desirable hydrogen storage material must have a high gravimetric and volumetric density, a suitable absorption/desorption temperature/pressure, good kinetics, good reversibility, resistance to poisoning by contaminants including those present in the hydrogen gas and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
Heat transfer capability can enhance or inhibit efficient exchange of hydrogen into and out of hydrogen storage metal alloys used in hydride storage systems. During hydriding of the hydrogen storage alloy an exothermic reaction occurs whereby hydrogen is absorbed into the hydrogen storage alloy and during dehydriding of the hydrogen storage alloy an endothermic reaction occurs whereby hydrogen is desorbed from the hydrogen storage alloy. In many instances, heat transfer within the hydrogen storage alloy utilized in the hydrogen storage systems cannot be relied upon for effective heat transfer within the hydrogen storage system since metal hydrides, in their hydrided state, being somewhat analogous to metal oxides, borides, and nitrides (“ceramics”), may be considered to be generally insulating materials. Therefore, moving heat within such systems or maintaining preferred temperature profiles across and through volumes of such storage materials becomes a crucial factor in metal alloy-metal hydride hydrogen storage systems. As a general matter, release of hydrogen from the crystal structure of a metal hydride requires input of some level of energy, normally heat. Placement of hydrogen within the crystal structure of a metal, metal alloy, or other storage system generally releases energy, normally heat, providing a highly exothermic reaction of hydriding or placing hydrogen atoms within the crystal structure of the hydrideable alloy.
The heat released from hydrogenation of hydrogen storage alloys must be removed. Heat ineffectively removed can cause the hydriding process to slow down or terminate. This becomes a serious problem which prevents fast charging. During fast charging, the hydrogen storage alloy is quickly hydrogenated and considerable amounts of heat are produced. The present invention provides for effective removal of the heat caused by the hydrogenation of the hydrogen storage alloys to facilitate fast charging of the hydride material.
Due to the heat input and heat dissipation needs of such systems, particularly in bulk, and in consideration of the insulating nature of the hydrided material, it is useful to provide means of heat transfer external to the storage material itself. Others have approached this in different ways, one by inclusion of a metal-bristled brush or brush-like structure within the hydrogen storage alloy, depending upon the metal bristles to serve as pathways for effective heat transfer. Another has developed a heat-conductive reticulated open-celled “foam” into which the hydrided or hydrideable material is placed.
Another recognized difficulty with hydride storage materials is that as the hydrogen storage alloy is hydrided, it will generally expand and the particles of storage material will swell and, often crack. When hydrogen is released, generally on application of heat, the storage material or hydrided material will shrink and some particles may collapse. The net effect of the cycle of repeated expansion and contraction of the storage material is comminution of the alloy or hydrided alloy particles into successively finer grains. While this process may be generally beneficial to the enhancement of overall surface area of the alloy or storage material surface area, it creates the possibility that the extremely fine particles may sift through the bulk material and settle toward the lower regions of their container or shift by gas flow and pack more tightly than is desirable. The highly packed localized high density region may produce a great amount of strain on the vessel due to the densification and expansion (upon charging) of the hydrogen storage material. The densification and expansion of the hydrogen storage material provide the possibility of deformation, cracking, or rupture of the container in which the hydrideable material is stored. While pressure relief devices may be useful in preventing such undesired occurrences as the container rupture due to the internal gas pressure of the vessel, pressure relief devices are unable to prevent deformation of the vessel resulting from densification and expansion of the hydrogen storage alloy. Others have approached the problem by dividing the container into simple compartments in a manner that prevents collection of too many fines in a particular compartment while allowing free flow of hydrogen gas throughout the container.
While including heat transfer and/or compartmentalization structures in a metal hydride hydrogen storage system has many benefits, the inclusion of such structures is not without problems. The heat transfer and/or compartmentalization structures, due to their size with respect to allowable vessel openings, can be difficult to properly position into prefabricated seamless pressure containment vessels. As such, prefabricated vessels are not typically utilized for hydrogen storage units containing such structures. A two piece pressure containment vessel may be used to house the hydrogen storage alloy, however, after the heat transfer/compartmentalization structures are placed inside the two pieces and the two pieces are welded together to form the vessel, a seam is formed which may provide weakness to the vessel structure. To place the heat transfer/compartmentalization structures within a seamless pressure containment vessel, a pressure containment vessel may be formed around the heat transfer/compartmentalization structures utilizing a spinning process, but this process can be timely and may increase the production cost of the system. The ability to purchase prefabricated pressure containment vessels in bulk then place the heat transfer/compartmentalization structures within the prefabricated vessels can be a cost effective way of constructing metal hydride hydrogen storage units and is highly desirable.